The present invention relates generally to an illumination apparatus, and more particularly to an illumination apparatus that illuminates a patterned mask (or a reticle) in an exposure apparatus in a lithography process used to manufacture a semiconductor device, a liquid crystal device, an image-pickup device (such as a CCD), a thin film magnetic head, etc. The present invention is suitable, for example, for an illumination apparatus that illuminates a reticle at a predetermined polarization state in an exposure apparatus having a projection optical system having such a high numerical aperture (“NA”) as 0.9 or higher.
A projection exposure apparatus is employed and uses a projection optical system to project a circuit pattern of a reticle onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology.
The minimum critical dimension (“CD”) transferable by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the NA of the projection optical system. The shorter the wavelength is, the better the resolution is.
A demand for finer semiconductor devices promote use of a shorter wavelength of the exposure light and the higher NA of the projection optical system. Recently, the exposure light source has shifted from the ultra-high pressure mercury lamps (g-line with a wavelength of approximately 436 nm and i-line with a wavelength of approximately 365 nm) to those having shorter wavelengths, such as a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm). A practical use of F2 laser (with a wavelength of approximately 157 nm) is also advanced. On the other hand, for a higher NA of the projection optical system, a projection optical system having the NA of about 0.6 is dominant in the 1990s, but the development of a projection optical system having a NA higher than 0.9 is now expected. The immersion exposure technology that immerges at least part of the projection optical system and the wafer with a higher refractive index material (liquid, such as a water) than the air provides a projection optical system with a NA of 1.0 or higher. See, for example, Japanese Patent Application, Publication No. 10-303114.
It is necessary for the projection exposure apparatus to control the exposure dose used to expose a pattern to a predetermined one suitable for the resist or a photosensitive material, or to monitor and control the exposure dose during exposure to the wafer. Thus, the projection optical system splits the exposure light using a half mirror, and monitors the exposure dose during the exposure using a exposure dose sensor. See, for example, Japanese Patent Applications, Publication Nos. 2000-294480 and 2000-277413.
Problematically, the p-polarized light lowers the contrast of the interference fringe in the resist applied to the wafer, as a NA of the projection optical system increases. This is because the resist exposes due to the light intensity of the electric field component of the light: The electric field vector of the p-polarized light does not generate an interference fringe, but provides a uniform light intensity distribution irrespective of locations.
FIG. 7 is a view showing that the contrast of the interference fringe (or image) reduces in a high-NA projection optical system. The XYZ coordinate is set as shown in FIG. 7, and diffracted lights E+ and E− interfere with each other and form the interference fringe. Each diffracted light contains s-polarized light Es having an electric field vector parallel to a substrate PL, and p-polarized light Ep orthogonal to the s-polarized light Es. The diffracted lights E+ and E− are expressed by the following Equations 1 and 2, where ν is a frequency and λ is a wavelength. However, for simple description purposes, the s-polarized and p-polarized lights have the same phase or are linearly polarized lights in the 45° direction.
                              E          +                =                              (                                                                                -                    Ep                                                                    cos                                                  θ                                                                              Es                                                                                                                                                                                                                                                                                  -                    Ep                                                                    sin                                                  θ                                                      )                    ⁢                      ⅇ                          2              ⁢              π              ⁢                                                          ⁢                              ⅈ                ⁡                                  (                                      vt                    -                                                                  z                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                            λ                                        +                                                                  x                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                        θ                                            λ                                                        )                                                                                        [                  EQUATION          ⁢                                          ⁢          1                ]                                          E          -                =                              (                                                                                -                    Ep                                                                    cos                                                  θ                                                                              Es                                                                                                                                                                                                                                                                                  -                    Ep                                                                    sin                                                  θ                                                      )                    ⁢                      ⅇ                          2              ⁢              π              ⁢                                                          ⁢                              ⅈ                ⁡                                  (                                      vt                    -                                                                  z                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                            λ                                        -                                                                  x                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                        θ                                            λ                                                        )                                                                                        [                  EQUATION          ⁢                                          ⁢          2                ]            
A sum of these vectors is a wave front of the interference fringe, and given by the following Equation 3:
                                          E            +                    +                      E            -                          =                              (                                                                                                      -                      2                                        ⁢                    Ep                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                    θ                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                                          (                                              2                        ⁢                        π                        ⁢                                                                              x                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                            θ                                                    λ                                                                    )                                                                                                                                        2                    ⁢                    Es                    ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                                          (                                              2                        ⁢                        π                        ⁢                                                                              x                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                            θ                                                    λ                                                                    )                                                                                                                                                              -                      2                                        ⁢                    iEp                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    θ                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                                          (                                              2                        ⁢                        π                        ⁢                                                                              x                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                            θ                                                    λ                                                                    )                                                                                            )                    ⁢                      ⅇ                          2              ⁢              π              ⁢                                                          ⁢                              ⅈ                ⁡                                  (                                      vt                    -                                                                  z                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                        θ                                            λ                                                        )                                                                                        [                  EQUATION          ⁢                                          ⁢          3                ]            
A square of an absolute value of this wave front (Equation 3) is the light intensity of the interference fringe, and given by the following Equation 4:
                                                                                                                                                      E                      +                                        +                                          E                      -                                                                                        2                            =                            ⁢                                                4                  ⁢                                      Ep                    2                                    ⁢                                      cos                    2                                    ⁢                  θ                  ⁢                                                                          ⁢                                                            cos                      2                                        ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                              x                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                            θ                                                    λ                                                                    )                                                                      +                                                                                                      ⁢                                                4                  ⁢                                      Es                    2                                    ⁢                                                            cos                      2                                        ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                              x                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                            θ                                                    λ                                                                    )                                                                      +                                                                                                      ⁢                              4                ⁢                                  Ep                  2                                ⁢                                  sin                  2                                ⁢                θ                ⁢                                                                  ⁢                                                      sin                    2                                    ⁡                                      (                                          2                      ⁢                      π                      ⁢                                                                        x                          ⁢                                                                                                          ⁢                          sin                          ⁢                                                                                                          ⁢                          θ                                                λ                                                              )                                                                                                                          =                            ⁢                                                4                  ⁢                                      (                                                                  Es                        2                                            +                                                                        Ep                          2                                                ⁢                        cos                        ⁢                                                                                                  ⁢                        2                        ⁢                        θ                                                              )                                    ⁢                                                            cos                      2                                        ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                              x                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                            θ                                                    λ                                                                    )                                                                      +                                                                                                      ⁢                              4                ⁢                                  Ep                  2                                ⁢                                  sin                  2                                ⁢                θ                            ⁢                                                                                                      [                  EQUATION          ⁢                                          ⁢          4                ]            
A coefficient
      cos    2    ⁡      (          2      ⁢      π      ⁢                        x          ⁢                                          ⁢          sin          ⁢                                          ⁢          θ                λ              )  in Equation 4 corresponds to the interference fringe, and provides a light intensity distribution of the line and space (“L & S”) in the x direction. When a fine pattern is projected by the projection lens or optical system having a high NA, an angle θ between diffracted lights increases. For example, FIG. 8 shows the angle θ between diffracted lights in the resist (with a refractive index of 1.7) when the L & S having a period L nm is projected with a wavelength of 193 nm of the ArF excimer laser. Referring to FIG. 8, the angle θ between diffracted lights is 45° for a binary mask when the period of the L & S becomes smaller than 160 nm, and for a Levenson phase shift mask (“PHM”) (or alternating-PSM) when the period of the L & S becomes smaller than 80 nm.
When the angle θ between diffracted lights becomes 45°, a term of cos2θ becomes 0 and the p-polarized light does not contribute to the amplitude of the interference fringe, lowering the contrast of the interference fringe of sin2θ. The p-polarized and s-polarized lights are defined by a relationship between the diffracted light and the substrate PL. While the above description refers to the p-polarized and s-polarized lights for a pattern that extends in the x direction (“x pattern”), the diffracted light occurs in the y direction for a pattern that extends in the y direction (“y pattern) so that the s-polarized light has an electric field vector in the x-axis direction and the p-polarized light is orthogonal to the s-polarized light. In other words, the s-polarized light for the x pattern corresponds to the p-polarized light for the y pattern. Notably, the polarization state varies according to a base surface and a light incident direction.
Thus, the p-polarized light lowers the image contrast in an exposure apparatus having a high-NA projection optical system. In order to obtain a high-contrast image, it is necessary to reduce the p-polarized light and use the polarization controlled light, i.e., the s-polarized light for exposure.
One problem for exposure with the polarization controlled light is that a half mirror that splits the exposure light has different transmittances between the p-polarized and s-polarized lights. Even when an illumination optical system forms desirably polarized light, the polarization state of the light that transmitted through the half mirror cannot become a desired state, deteriorating the resolution.
FIG. 9 shows transmittances of the s-polarized and p-polarized lights through a non-coated surface (a surface that has no antireflection coating). A conventional exposure apparatus reflects the exposure light on a half mirror and directs the reflected light to an exposure dose sensor, while inclining the half mirror by about 40° to the optical axis so that the exposure dose sensor does not shield the exposure light. For example, the exposure light generally has an angular distribution between about ±30°, and an angle incident upon the half mirror ranges between +10° and +70°. Referring to FIG. 9, the light incident upon the half mirror at the angle of +10° has a transmittance of about 95% for both the p-polarized and s-polarized lights, whereas the light incident upon the half mirror at the angle of +70° has a transmittances of 96% for the p-polarized light and a transmittance of 68% for the s-polarized light. When the s-polarized light is used for exposure, the light intensity differs as large as 27% according to angles incident upon the half mirror. The angularly dependent light intensity difference is as influential as the coma to the projection lens, and deteriorates the resolution.
In exposing a pattern that blends the x and y patterns, suppose that the s-polarized light to the y pattern is the s-polarized light for the half mirror surface and the s-polarized light to the x pattern is the p-polarized light for the half mirror surface. As a result, the light intensity that contributes to imaging in the x direction is greater than the light intensity that contributes to imaging in the y direction. This causes a contrast difference between the x and y patterns, and a HV difference or a difference of the resolved CD between the x and y patterns. This error problematically causes inaccurate pattern transfers.